How can we tell if our dometic rooftop air conditioner/furnace has a heat strip or not.

Any dealer should be able to order for you. Here's dometic site to locate nearest dealer to you.
https://www.edometic.com/allusr/srvctr.nsf/dl?OpenView

i would say its the relay board or communication cabl between stat and board.

Please give make, model, year etc of RV that furnace is in. Thermostat on wall operates heat and cool?

Open circuit or out of range Freeze Sensor. Air conditioner mode of operation will be locked out. Furnace, heat strip, heat pump and fan mode of operation can continue to operate but displays the last temperature set-point. There is a sensor that detects evaporator freezing, it may be bad or your freezing because of low freon levels

I hope this doesn,t sound patronising but i have seen people turn the themostat down to make the furnace come on obviously the thermostat has to be turned up to set for a higher tempreture , after a small time delay, should then light up.

E5 Open circuit or out of range Freeze Sensor. Air conditioner and dehumidification

operation will be locked out. Heat pump, furnace, heat strip and fan operation
can continue to operate but displays the last temperature set-point.

Sounds like your freeze sensor is defective and since it is still under warranty, you would void it if you replace it yourself.

You can get the heat strip at your local heating air distributer try looking for cc dickson company you will need the model number and serial number

Odds are u did just that or wrong thermostat,If u don't have a digrahm get one from Library.

i would suggest that you look at your thermostat,it may be to close to the furnace keeping in mind that all thermostats have a certain tempreture drop (or gain) before restarting john